Injection molding is one of the most important and efficient manufacturing techniques for polymeric materials, with the capability to mass produce high value added products, such as the compact disc. Injection molding can be used for molding other materials, such as thermoset plastics, ceramics and metal powders. The process in its present form was developed in the mid 1950s, when the first reciprocating screw machines became available. Material, machine and process variations are important in this complex multi-variable process. There are three interacting domains for research and development: 1) polymeric material technology: introduction of new and improved materials; 2) machine technology: development of machine capability; and 3) processing technology: analysis of the complex interactions of machine and process parameters. As improved product quality and enhanced engineering properties are required of polymeric materials, the injection molding process has become increasingly complex: as service properties increase material processability tends to decrease.
Thermoplastics can be classified as bulk or engineering materials. Engineering materials are typically more difficult to process, and more expensive, and therefore their processing would benefit the most from automated molding optimization (AMO). Injection molding is a batch operation, so machine set-up ultimately affects productivity.
Any molding operation should aim to manufacture component products to a specific quality level, in the shortest time, in a repeatable and fully automatic cycle. Injection molding machines usually provide velocity control and pressure control, that is, control of the velocity of the injection screw when filling the part and control of the pressure exerted by injection screw when packing/holding the part, respectively. The following description assumes the use of a modern injection molding machine, after circa 1980, with velocity control of the mold filling and pressure control of the packing/holding stages.
The typical injection molding cycle is as follows:                1) Plasticisation Stage: plasticisation occurs as the screw rotates, pressure develops against the ‘closed-off’ nozzle and the screw moves backwards (‘reciprocates’) to accumulate a fresh shot (the molten polymer in front the screw), ready for injection of melt in front of the screw tip. Back pressure determines the amount of work done on the polymer melt during plasticisation. Polymer melt is forced through the screw non-return valve. Material is fed to the screw by gravity from a hopper. The polymeric material may require conditioning, especially in the case of engineering thermoplastics, to ensure melt homogeneity and therefore that the melt has consistent flow characteristics.        2) Injection/Filling Stage: the empty mold is closed, and a ‘shot’ of polymer melt is ready in the injection unit, in front of the screw. Injection/filling occurs, polymer melt is forced though the nozzle, runner, gate and into the mold cavity. The screw non-return valve closes and prevents back-flow of polymer melt. In this, the mold filling part of the injection molding cycle, high pressures of the order of 100 MPa are often required to achieve the required injection velocity.        3) Packing/Compression Stage: a packing pressure occurs at a specified VP or ‘switch-over’ point. This is the velocity control to pressure control transfer point, i.e. the point at which the injection molding machine switches from velocity control to pressure control. ‘Switch-over’ should preferably occur when the mold cavity is approximately full, to promote efficient packing. The switch-over from injection to packing is typically initiated by screw position. Switch-over can be initiated by pressure, i.e. hydraulic, nozzle melt injection pressures or cavity melt pressure parameters measured from the machine. The end of this stage is referred to as ‘pack time’ or ‘packing time’.        4) Holding Stage: a second stage pressure occurs after the initial packing pressure and is necessary during the early stages of the cooling of the molded part to counteract polymer contraction. It is required until the mold gate freezes; the injection pressure can then be released. This phase compensates for material shrinkage, by forcing more material into the mold. Typical industrial machine settings use one secondary pressure, combining the packing and holding phases, to allow for easier machine set-up. It has been shown that under packing results in premature shrinkage, which may lead to dimensional variation and sink marks. Over packing may cause premature opening of the tool (i.e. the die or mold of the component(s) to be manufactured) in a phenomenon known as flashing, difficulties in part removal (sticking) and excessive residual stresses resulting in warpage. Analysis of the packing phase is therefore an essential step in predicting the final product quality. The portion of filling after switch-over can be more important than the velocity controlled primary injection stage. The end of this stage is known as ‘hold time’ or ‘holding time’.        5) Cooling Stage: This phase starts as soon as the polymer melt is injected into the cavity. The polymer melt begins to solidify when in contact with the cavity surface. Estimating cooling time is becoming increasingly important, especially when large numbers of components are being molded. In order to calculate cooling time, component ejection temperature should be known. Cooling an injection molded product uniformly may mean cooling the mold at different rates, in different areas. The aim is to cool the product as quickly as possible, while ensuring that faults such as poor surface appearance and changes in physical properties are not encountered. The aims for a cooling system are: (i) minimum cooling time, (ii) even cooling on part surfaces, and (iii) balanced cooling between a core and a cavity part of a two-plate tool system. Tool temperature control is required to maintain a temperature differential ΔT between the tool and the polymer melt. For example, a typical polyoxymethylene melt temperature is 215° C., tool temperature is 70° C., and hence ΔT=145° C. Adverse effects to product quality must be expected for no or poor temperature control. The cooling phase enables the polymer melt to solidify in the impression, owing to the heat transfer from the molded product to the tool. The tool temperature influences the rate at which heat is transferred from the polymer melt to the tool. The differences in heat transfer rate influence polymer melt shrinkage, which in turn influences product density. This effect influences product weight, dimensions, micro-structure and surface finish. The tool cavity surface temperature is critical to the processing and quality of injection molded components. Each part of the product should be cooled at the same rate, which often means that non-uniform cooling must be applied to the tool. Thus, for example, cool water should be fed into the inner parts of the tool cooling system (particularly in the area of the gate) and warmer water should be fed into the outer parts. This technique is essential when molding flat components to close tolerances, or large components that include long melt flow lengths from the gating position. Tool design must thus preferably incorporate adequate temperature control zones (flow ways), to provide the desired tool temperature. Tool temperature control zones commonly use water for temperatures up to 100° C., above which oil or electrical heating is used.        
Injection molding is one of the most sophisticated polymer processing operations, with machine costs typically ranging from US$50,000 to well over US$1,000,000 and tool costs ranging from $10,000 to well over $100,000. The vital operation of tool set-up is often not given the attention it deserves. If a machine is poorly set-up, then this will affect the cost of production, through cycle time and part rejection rates. Machine set-up is still regarded as a black art, reliant on the experience of a manual die setter (i.e. the person responsible for setting parameters on the injection molding machine to achieve acceptable quality production). In a typical injection molding manufacturing facility machine set-up is often overlooked with the requirement to ‘get parts out the door’. In this rush machine set-up is often done with inconsistent strategies as different die setters have their own personal views as to what constitutes an optimal set-up. Manufacturing facilities typically have a high staff turn-over on the shop floor, and so training and maintaining an adequate level of experience is also a high cost.